Constrained layer, composite, acoustic damping material

ABSTRACT

A constrained layer, composite structure for damping acoustic vibrations includes an extensional layer comprised of a first polymeric material, and a constraining layer of a second polymeric material. The modulus of elasticity of the constraining layer is greater than that of the extensional layer. In use, the structure is disposed on the surface of an article in which acoustic vibrations are to be damped so that the extension layer overlies the surface. Also disclosed are methods for preparing the structure, including automated methods.

RELATED APPLICATION

This application is a continuation-in-part of U.S. patent application Ser. No. 11/207,160 filed Aug. 18, 2005 (now abandoned), which claims priority of U.S. Provisional Patent Application Ser. No. 60/602,811 filed Aug. 19, 2004, entitled “Constrained Layer, Composite, Acoustic Damping Material.”

FIELD OF THE INVENTION

This invention relates generally to structures for damping acoustic vibrations. More specifically, the invention relates to composite damping structures fabricated from organic polymeric materials.

BACKGROUND OF THE INVENTION

Motor vehicles, electrical appliances, heating, ventilating and air conditioning systems, and similar structures include sheet metal members. The sheet metal members are very efficient in transmitting acoustic vibrations and, in many instances, can be a source of acoustic vibrations; hence, systems including sheet metal members often present significant noise control problems. Consequently, sound absorbing materials are often included in such structures for purposes of noise control.

Sheet metal components of motor vehicles frequently have sound absorbing patches applied thereto. These patches include an adherent, vibration dampening material. One embodiment of such patches comprises an adhesively adherent body of a rubbery polymeric material having a metal foil coating on one face thereof. The rubbery material is adhered to portions of a motor vehicle body, such as the inner surfaces of door panels, roof panels, and the like.

Another embodiment of sound deadening patches known in the art comprises dual layered structures in which polymeric layers having different viscosities are disposed in a stacked relationship. Patches of this type are shown, for example, in U.S. Pat. No. 5,227,592 of Kosters and U.S. Pat. No. 4,346,782 of Bohm. The patch structures in Kosters and Bohm are comprised of plastisol layers based on PVC or on metbacrylates. In the instance of the Kosters patent, at least one of the layers is foamed. It has been found that patch structures of this type suffer from material limitations insofar as the PVC and/or acrylic based layers do not adhere well to oily metal surfaces, and hence are not compatible with conditions encountered in the manufacture of sheet metal components which thereby limits their use and/or requires special cleaning and handling steps. Also, foamed materials can absorb liquids, such as processing fluids, and/or liquids encountered in ambient storage or use of the articles; and, these absorbed liquids can cause corrosion, and can interfere with subsequent processing operations. In addition, the polymeric materials used in these prior art patches have a relatively high glass transition temperature and hence are prone to cracking, delamination, or other damage under low temperature, high shock conditions.

Prior art sound deadening patches typically must be affixed to the vehicle following various assembly and processing operations, since patches of this type cannot generally survive operations such as dip-coating, painting, heating, and the like. Consequently, these patches must be applied during late stages of the vehicle's assembly when the locations to which the patches are to be applied are not readily accessible to robotic applicators or other such mechanized systems; therefore, these patches must be manually applied. Manual application is expensive, particularly when locations are ergonomically difficult to access. As a consequence, patch placement is often inaccurate; hence, the industry typically employs oversized patches to assure that proper coverage of vibrational nodes is achieved. These oversized patches increase material cost, and manual application increases the cost of labor. Also, manual placement frequently results in improper adhesion of the patch to the metal article. As a result, some, or all, portions of the patch may delaminate from the surface, thereby resulting in loss of sound deadening ability.

The finished appearance of the surface of a motor vehicle or other article of manufacture is usually very important. One problem that has been found with regard to prior art sound deadening patches is termed “read through”. Read through occurs when a patch disposed on the rear face of a sheet metal structure, such as a vehicle door, causes a perturbation on the front surface which causes irregularities in the appearance of the front surface. This problem can be particularly severe when prior art patches are applied prior to the time that painting or other surface finishing operations are carried out.

It will be appreciated from the foregoing that there is a need for improved materials and methods for providing sound deadening in motor vehicles and other such sheet metal structures. As will be explained in detail hereinbelow, the present invention provides a composite sound deadening patch which is highly efficient in function, not prone to produce read through, and is amenable to automated application techniques. The materials comprising the patch of the present invention may be dispensed in a paste or semisolid, pumpable form which allows for use of high-speed, robotically controlled coating techniques. The patch structure of the present invention does not cause read through and provides superior acoustic attenuation. Furthermore, the materials comprising the patch are compatible with processing techniques such as painting, dip-coating, oven baking, and the like. Hence, the patch of the present invention may be applied at early stages in the manufacturing process, thereby reducing material and labor costs. These and other advantages of the invention will be apparent from the drawings, discussion and description which follow.

BRIEF DESCRIPTION OF THE INVENTION

Disclosed herein is a constrained layer, composite structure for damping acoustic vibrations in an article. The composite structure includes an extensional layer of a first polymeric material. The first polymeric material has a first modulus of elasticity. In the use of the composite structure, the extensional layer is disposed upon a surface of an article in which acoustic vibrations are to be damped. The structure further includes a constraining layer of a second polymeric material which has a modulus of elasticity which is greater than the modulus of elasticity of the first polymeric material. The constraining layer is disposed atop the extensional layer. In some instances, the constraining layer may cover all of the extensional layer, while in other instances it may cover only a portion of the extensional layer.

In particular instances, the modulus of elasticity of the constraining layer is at least one order of magnitude greater than that of the modulus of elasticity of the material comprising the extensional layer. The first polymeric material may comprise a viscoelastic polymer. In particular instances, at least one of the polymeric materials is a thermally curable material, and these thermally curable materials may be nonhydroscopic and/or pumpable prior to being thermally cured.

The first polymeric material may comprise a mixture of synthetic rubber, a tackifier, an epoxy resin, a vinyl resin, and a thermally activated crosslinker. The second polymeric material may comprise a mixture of an epoxy resin, an elastomeric modifier, a tackifier, and a thermally activated crosslinker. One or more of the polymeric materials may include a flexibilizing reactive diluent, an inorganic filler, a reinforcing fiber, a foaming agent, and/or hollow microspheres.

In particular instances, the thickness of the extensional layer is in the range of 1-6 mm, while in some particular instances, the thickness of this layer is in the range of 2-4 mm. The thickness of the constraining layer may be in the range of 0.5-3 mm, and in particular instances in the range of 0.5-1.0 mm. In specific instances, the thickness of the extensional layer is greater than the thickness of the constraining layer. In particular embodiments, the loss factor of the composite structure is greater than 0.1 for frequencies in the range of 200-800 Hz.

Disclosed are some specific compositions for the extensional layer and constraining layer, which interact to provide a sound deadening composition which is vey effective over a wide range of frequencies. This particular compositions adhere very well to oily metal, are stable at both high and low temperatures, and do not promote corrosion of metallic materials upon which they are disposed. The materials of this composition do not include ant PVC resins, and the structures do not incorporate any foamed polymers.

Also disclosed herein are methods for the manufacture of the constrained layer, composite, acoustic vibration damping structures of the present invention. These methods may include use of coating techniques such as extrusion coating, shoveling, spray coating, and swirl coating, either singly or in combinations for the deposition of one or more of the layers. In specific instances, one or both of the layers may be deposited by a robotic coating apparatus.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a cross-sectional view of a substrate having the composite acoustic dampening structure of the present invention applied thereto.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to a constrained layer, composite structure used for damping acoustic vibrations in articles such as motor vehicle body panels and other sheet metal structures. The damping material of the present invention includes at least two layers of polymeric material; and referring now to FIG. 1, there is shown one embodiment of a composite acoustic damping structure of the present invention 10 as disposed upon a substrate 12 which may comprise a body of sheet metal in which acoustic vibrations are to be dampened. As will be seen, the acoustic damping structure 10 includes a first layer 14, also referred to as an extensional layer, which is disposed upon the substrate 12, and a second layer 16, also referred to as a constraining layer, which is disposed atop the first layer.

The first layer 14 and second layer 16 are both comprised of polymeric materials. The polymeric material of the first layer 14 is relatively soft compared to the material of the second layer 16. As such, the polymeric material of the second layer has a modulus of elasticity which is greater than that of the first layer 14. This combination of layers gives a high degree of acoustic and vibrational damping, and has also been found to increase the flexural strength of a panel to which it is attached. In specific instances, the modulus of elasticity of the second layer 16 is at least an order of magnitude greater than that of the first layer 14, and in particular instances, the modulus of the second layer 16 is at least several orders of magnitude greater than that of the first. It has been found that this combination of materials produces a synergistic effect which greatly enhances the ability of the material to attenuate acoustic vibrations over a wide range of frequencies.

In specific instances, the relative thicknesses of the two layers 14 and 16 are selected so that the first layer is thicker than the second layer 16. Acoustic damping generally increases as the softness and/or thickness of the first layer increases. Damping increases to a lesser degree with increasing thicknesses of the second layer. In particular embodiments, the thickness of the first layer 14 is in the general range of 1-6 mm. In some particular instances, the thickness of the first layer is in the range of 2-6 mm, while in other instances it is in the range of 2-4 mm. Typical thickness ranges for the second layer are 0.5-3 mm, and one particular range is 0.5-1.0 mm.

There are a variety of polymeric materials which may be employed in the practice of the present invention. In one specific group of embodiments, the polymeric material of the first layer 14 is comprised of a viscoelastic polymer. As is known in the art, viscoelastic polymeric materials have mechanical properties such that when a deforming force is applied thereto, they respond with hybrid characteristics corresponding to a resilient, spring-like component, as well as a viscous damping, dashpot-like component. There are a variety of polymeric materials known in the art which provide this characteristic behavior. The second material is of a more rigid nature. While not wishing to be bound by speculation, the inventors hereof assume that this combination of layers traps and attenuates acoustic vibrations in a very efficient manner.

In particular embodiments of the present invention, the polymeric materials comprising the two layers are thermally cross-linkable polymers which may be applied in a first, relatively soft, viscous form and subsequently cured by the application of heat to produce a harder material. In such instances, the various materials may be readily applied by automated bulk coating techniques. For example, uncured coatings may be applied to the articles by extrusion techniques, spray techniques, swirl techniques, or the like.

In one specific embodiment, coatings are readily applied by a technique called “shoveling.” This technique is an extrusion-type coating process in which material is dispensed as a sheet, directly onto the surface being coated. Such automated techniques may be employed for the deposition of one or both of the layers. In those instances where curable coatings are employed, these coatings may be cured right after application, or they may be subsequently cured, concomitant with further processing of the article. For example, the coatings may be dispensed onto a shaped body panel which is subsequently primed or painted, and the coating will be cured when the paint or primer is oven baked. In specific embodiments of the present invention, the compositions of the layers are formulated so that the uncured polymeric materials are non-hydroscopic, stable and adherent to the coated article. In this manner, the steps of applying the sound attenuating layers may be implemented early on in the manufacturing process, thereby simplifying placement and application of the materials. The mechanical and environmental stability of the uncured coatings allows for delayed cure without loss of properties.

In some particular embodiments, the material comprising the first, extensional layer includes compositions of a synthetic rubber, a tackifier, an epoxy resin, and a thermally activated crosslinker. In some instances, the composition may also include a vinyl resin, which may be a curable vinyl resin.

The composition of the constraining layer, in some instances, may comprise an epoxy resin, an elastomeric modifier, a tackifier, and a thermally activated crosslinker. The second composition may also include a flexibilizing active diluent as well as ancillary ingredients such as an inorganic filler, reinforcing fibers such as glass fibers, carbon fibers, or polymeric fibers. Either of the layers may also include a foaming agent, hollow microspheres of glass, ceramic, or a polymer, and other such ancillary ingredients.

The compositions of the present invention show very good sound attenuation over a wide frequency range. In general, the structures of the present invention exhibit a loss factor which is greater than 0.1, as measured at frequencies of 200, 400 and 800 Hz. This is considered to very good sound attenuation. In particular instances, loss factor values of 0.24-0.25 have been demonstrated utilizing relatively thin layers configured in accord with the present invention.

The materials of the present invention are compatible with a wide variety of substrates and have been demonstrated to have very good adherence to steel substrates at temperatures as low as −35° C., as was verified by standardized Cold Slam and Cold Bend tests. In addition to providing very good acoustic damping, it has been found that the materials of the present invention enhance the corrosion resistance of articles to which they are applied. The layers are resistant to metal treatment fluids and other such oily materials which are typically encountered in the fabrication of motor vehicles and the like. Such fluids include phosphate and E-coat conditioners. Hence, these compositions can be applied to articles prior to surface finishing steps such as painting and the like. “Read through” is a problem typically encountered when structures are applied to the backside of painted, or to be painted, panels. Read through results when the presence of the applied article alters various characteristics of the panel, such as expansibility, thermal conductivity, or the like so that a paint film applied to the opposite face of the article is distorted in a corresponding pattern. It is notable that the acoustic damping structures of the present invention do not produce read through.

In view of the teaching presented herein, various combinations of materials may be readily selected by one of skill in the art so as to produce effective, constrained layer, acoustic damping structures. Some specific examples of materials which may be used to form the extensional and constraining layers of the present invention are as follows.

A first embodiment of extensional layer material comprises, on a weight percentage basis, 15% of a diglycidyl ether of bisphenol A such as the material sold under the designation NPEL 128E by the Nanya corporation; 10% asphalt such as the material sold under the designation Zeco 9000 by the Ziegler corporation; 30% calcium carbonate such as the material sold under the designation #10 white by the Imerys corporation; 5.2% mica such as the material sold under the designation mica 150 S by the Sutrite corporation; 15% styrene-butadiene rubber such as the material sold under the designation 1009 SBR by the ISP corporation; 0.1% carbon black such as the material sold under the designation Raven 410 by the Columbian corporation; 2.1% wollastonite such as the material sold under the designation Nyad M200 by the Nyco corporation; 1% fumed silica such as the material sold under the designation Cabosil PTG by the Cabot corporation; 1% dicyandiamide such as the material sold under the designation Dyhard 100S by the Degussa corporation; 1.5% of cashew nutshell liquid polymer with epichlorohydrin such as the material sold under the designation Lapox RA913 by the Royce corporation; 3% of a trifunctional epoxidized castor oil such as the material sold under the designation GE 35 by CVC Chemicals; 6% of an adduct of dimer acid and diglycidyl ether of bisphenol A such as the material sold under the designation Hypox RA 323 by CVC Chemicals; and 10% of a hydrocarbon tackifier resin such as the material sold under the designation Piccotac 1102 by the Eastman corporation.

A second formulation for the extensional layer comprises, on a weight basis, 2% of a liquid EPDM resin such as the material sold under the designation Trilene 65 by the Crompton corporation; 15% of the aforementioned asphalt; 18.4% of the aforementioned calcium carbonate; 15% of the aforementioned mica; 2% of the aforementioned wollastonite; 10% of talc such as the material sold tinder the designation Veltal 97 by the Luzenac corporation; 0.1% of the aforementioned carbon black; 1% of fumed silica; 2.5% of PVA (polyvinyl acetate) resin; 3% of zinc diacrylate such as the material sold under the designation SR 633 by the Sartomer corporation; 6% of a diisononyl phthalate plasticizer having a molecular weight of approximately 420 such as the material sold under the designation 9P plasticizer by the BASF corporation; 10% of the aforementioned diglycidyl ether of bisphenol A; a rubber such as the material sold under the designation Taktene 100 by the LanXess corporation.

A third formulation of a material which may be used to form an extensional layer comprises, on a weight basis, 5% of the aforementioned liquid EPDM resin; 11.3% of the aforementioned asphalt; 25% calcium carbonate; 15% mica; 2% wollastonite; 20% of a polybutene resin such as the material sold under the designation Indopol H100 by the BP corporation; 0.1% carbon black; 0.1% fumed silica; 5.5% of the aforementioned zinc diacrylate; 6% of the aforementioned diisononyl phlithalate plasticizer; and 10.3% of butyl rubber such as the material sold under the designation Butyl 065 by Exxon Mobil.

A fourth example of material which may be used for the extensional layer comprises, on a weight percentage basis, 2% of crosslinked butyl rubber such as the material sold under the designation Kalar 5215 by the Elementis corporation; 3% of the aforementioned butyl rubber; 5% of a C5 tackifier resin such as the material sold under the designation Escorez 1102 by the Exxon Mobil corporation; 10% calcium carbonate; 10% mica; 30% of a calcium carbonate sold under the designation #10 white by the Inerys corporation; 30% of the aforementioned polybutene resin; 5% of a diluent such as the material sold under the designation OLO by the Gage Products Corporation; and 5% of the aforementioned diisononyl phthalate plasticizer.

A fifth material which may be used for the fabrication of the extensional layer comprises, on a weight basis, 5% of dipropylene glycol diacrylate such as the material sold under the designation SR 508 by the Sartomer corporation; 10% of isobomyl acrylate such as the material sold under the designation SR 506D by the Sartomer corporation; 10% tetrahydrofiufaryl acrylate such as the material sold under the designation SR 285 by the Sartomer corporation; 5% carboxylated nitrile rubber such as the material sold under the designation Nipol 1472 by the Zeon corporation; 10% of the aforementioned diisononyl plithalate plasticizer; 47% calcium carbonate; 10% mica; 2% fumed silica; and 1% of dicumyl peroxide such as the material sold by various sources under the name Dicup 40C.

Yet another composition useful as an extensional layer comprises, on a weight basis, 10% of a blocked isocyanate resin; 50% of polypropylene glycol having an approximate molecular weight of 2000; 35% calcium carbonate; and 5% of a thixotropic agent sold under the designation R202 by the Degussa corporation. Blocked isocyanates which may be used in the present invention are well known to those of skill in the art, and one particular composition which may be employed comprises, on a weight percentage basis, 35% polytetramethylene glycol; 25% diisodecyl phthalate; 15% MDI, methyldiisocyante, together with 0.1 ml of an agent sold under the designation Dabco T-12. This composition is heated for three hours at 80° C. under a nitrogen atmosphere, and cooled to 50°, and blended with 25% methyl ethyl ketoxime. Other compositions of blocked isocyanate may be likewise employed in this composition.

In view of the foregoing, yet other embodiments of material usable in the extensional layer will be readily apparent to those of skill in the art.

A first example of material which may be used for fabricating the constraining layer comprises, on a weight basis, 4% of an aromatic hydrocarbon resin such as the material sold under the designation Nevex 100 by the Neville corporation; 25.7% of the aforementioned diglycidyl ether of bisphenol A; 12.8% of a diglycidyl ether of polypropylene glycol such as the material sold under the designation NPEL 032 by the Nanya corporation; 6.5% of the aforementioned cashew nutshell liquid polymer with epichlorohydrin; 4.3% mica; 8.6% wollastonite; 30% calcium carbonate; 4.3% of a slag wool fiber such as the material sold under the designation PMF fiber by Sloss Industries; 0.4% titanium dioxide; 1.8% fumed silica; and 3.2% of a curing agent such as the material sold under the designation 100S by the Degussa corporation.

A second formulation for the constraining layer comprises, on a weight basis, 10% of the aforementioned diglycidyl ether of bisphenol A; 20% of a low Tg polyester resin such as the material sold under the designation Dynapol S 320 by the Degussa corporation; 10% of the aforementioned diglycidyl ether of polypropylene glycol; 5% of the aforementioned cashew nutshell liquid polymer with epichlorohydrin; 36% calcium carbonate; 15% mica; 3% amorphous silica; and 1% of a curing agent sold under the designation 100S by the Degussa corporation.

A third formulation for the constraining layer comprises, on a weight basis, 20% of a thermoplastic urethane resin such as the material sold under the designation Desmopan 9370 AR by the Bayer corporation; 20% of the aforementioned diglycidyl ether of bisphenol A; 10% of the aforementioned diglycidyl ether of polypropylene glycol; 5% of the aforementioned cashew nutshell liquid polymer with epichlorohydrin; 26% calcium carbonate; 15% mica; 3% amorphous silica; and 1% of the curing agent sold under the designation 100S by the Degussa corporation.

In view of the foregoing, yet other modifications and variations of formulations for the constraining layer will be apparent to those of skill in the art.

A series of representative composite structures of extensional (bottom) layer and constraining (top) layer were prepared and tested for sound loss factor at different thickness combinations, cure temperatures and test temperatures. The loss factors at 200 Hz, 400 Hz, and 800 Hz are shown in Table 1 and Table 2. The constraining layer can also serve as a reinforcement member for the scale panels. Typically, bare steel of 0.8 mm thickness has a flexural strength (N) of 40-45. When coated with a composite structure having a 2 mm thick constrained layer, its flexural strength increases to 50-55. When die thickness of the constrained layer is increased to 3 mm, the flexural strength is 60-65, and remains at that value if the thickness is increased to 4 mm.

The respective thicknesses of the constrained layer and extensional layer (also referred to as the soft layer) can be selected for particular applications in view of the teaching presented herein. Table 1 hereinbelow details loss factors at different frequencies, for damping structures of the present invention including layers of various thicknesses.

TABLE 1 Ratio Total of Soft/ Thickness Constraining Loss Factor (mm) Layers Cure Conditions 200 Hz 400 Hz 800 Hz 6.2 3:1 375° F. 20 min. 0.250 0.252 0.288 4.6 2:1 375° F. 20 min. 0.202 0.184 0.242 3.6 1:1 375° F. 20 min. 0.154 0.157 0.164 4.1 2:1 400° F. 40 min. 0.164 0.199 0.251 3.8 2:1 400° F. 40 min. 0.146 0.165 0.180 3.2 2:1 400° F. 40 min. 0.126 0.125 0.121

Table 2 summarizes vibration damping properties of a structure corresponding generally to the first entry of Table 1, as measured at various temperatures and frequencies. Measurements were conducted in accord with procedure SAE J1637.

TABLE 2 Test Temperature Resonant Frequency Hz Loss Factor −10° C.  200 0.036 400 0.041 800 0.050 10° C. 200 0.071 400 0.070 800 0.077 25° C. 200 0.250 400 0.252 800 0.288 40° C. 200 0.230 400 0.231 800 0.242 60° C. 200 0.156 400 0.208 800 0.274 80° C. 200 0.046 400 0.059 800 0.079

Further in accord with the present invention, Applicants have developed a particular group of formulations for composite acoustic damping structures. These formulations differ from sound dampening compositions and structures of the prior art insofar as they do not include any polyvinylcliloride (PVC) components and hence have an improved heat tolerance which allows them to be used prior to high temperature processing operations. Also, the compositions of the present invention have improved cold tolerance and adhesion as compared to prior art materials. The compositions of this embodiment of the invention utilize a mixture of rubber materials, and this allows for very good control of the glass transition temperature of the damping layers. Finally, the compositions of the present invention incorporate ethylene methacrylate copolymers (EMA) as well as rosin esters, and the inclusion of these materials enhances the ability of the compositions to adhere to oily metal substrates, and in particular to adhere in a vertical orientation. As a result of the foregoing, it has been found that compositions in accord with the present invention readily adhere to oily metal substrates such as steel substrates, aluminum substrates and the like; furthermore, the compositions maintain good adherence even in a vertical orientation. The present group of compositions also can tolerate high heat conditions of at least 200° C. This combination of properties makes this particular group of materials very well suited for use in the fabrication of motor vehicles, since the compositions may be applied to body panels and the like during early stages of the fabrication process, preferably by robotic applicators. The thus coated panels may then be subjected to further processing such as washing, electro coating, painting, baking and the like without causing a loss of adhesion or generating undesirable effects such as read through.

In a typical implementation, the first, extensional, layer will include a first component which is an epoxy resin and is present in a weight amount (as are all amounts given herein unless otherwise noted) of 3-10%. This epoxy resin may be a diglycidyl ether of bisphenol A. Materials of this type are available from a number of sources as for example the NPEL 128E material sold by the Nanya corporation as noted above. Other commercially available versions of this resin are sold under the designation EPON 828 and DER 331, among others. A second component comprises a rosin ester which is present in an amount of 3-10%. This material acts as a tackifier which serves to enhance the adhesion of the layer during processing, and particularly during washing steps. The extensional layer of this group of embodiments includes 5-12% of a styrene-butadiene rubber, and such rubbers are commercially available from a number of sources including the materials sold by the ISP corporation under the designation 1009 SBR. The composition includes a second rubber component, typically in the amount of 5-15%, and this component is a polybutadiene rubber. One such material is available under the designation Taktene 220. It has been found that use of the mixture of rubbers allows for better control of the glass transition temperature, T_(g), of the layer. The composition will also include a plasticizer material in an amount of 2-20%, and one such plasticizer comprises diisodecyl phthalate. Such materials are available from a number of sources including the Exxon corporation under the designation Jayflex DIDP. Other phthalate esters may likewise be utilized. The composition also includes an aromatic hydrocarbon material of the type known in the art as “process oil”. This process oil serves to masticate the rubber component, and is typically present in an amount of 2-20%. One commercially available process oil is sold under the designation Calsol 610. The composition also includes 1-5% of asphalt, and it has been found that inclusion of the asphalt enhances the moisture resistance of the composition and enhances the corrosion protection of metal upon which it is disposed. There are a number of asphalt materials which may be utilized for this purpose, and one such material is commercially available under the designation AA1956. The composition will also include a thermoplastic polyester resin in an amount of 1-5%, and one such resin comprises a polyester resin such as the resin sold under the designation Dynapol S 1606. A second thermoplastic component is also present in an amount of 1-5% and comprises an ethylene methacrylate copolymer such as the material sold under the designation Optema TC 141. This methacrylate material enhances the adhesion of the composition to oily metal. The composition may further include a crosslinker for the rubber. The crosslinker typically will comprise sulfur and will be present in an amount of 0.2-1%. Sulfur is available from a number of sources, and one particular material used in the present invention is sold under the trade name Alcrochem MC-98. The composition will further include a crosslinker for the epoxy component, and one such crosslinker comprises dicyandiamide such as the material sold under the designation Dyhard 100S. This component is present in an amount of 0.3-0.8%.

The extensional layer may further include inorganic materials which function as fillers and/or as materials for modifying the properties of the layer. Calcium carbonate is one such filler material and may be present in an amount of 10-40%. Barium sulfate can function as a filler and densifier and is present in an amount of 5-20%. Mica may also be included in the composition and the mica, in addition to functioning as a filler, further enhances the acoustic damping ability of the composition as a result of its plate structure. Mica is typically present in an amount of 5-15%, and one grade of mica having utility in the present invention is commercially available under the trade name Suzorite Mica 150S. The composition may also include fumed silica which can function to make tlie composition thixotropic prior to curing. Fumed silica is typically present in an amount of 1-4%, and one commercially available fumed silica having utility in the invention is Aerosil 202.

The second or top layer, also referred to as a constraining layer, may be compounded from a first ingredient which is an epoxy resin such as the bisphenol A diglycidyl ether used in the extensional layer. This epoxy component is typically present in an amount of 10-25%. The constraining layer can also include a powdered polymeric material such as acrylonitrile butadiene styrene (ABS) powder in an amount of 5-15%. Such material is commercially available under the designation Blendex 338. The constraining layer will further include an aromatic tackifier resin such as the coumarone indene resin sold under the designation Cumar 130. Other aromatic taclcifier resins may likewise be employed. This component is typically present in an amount of 0.5-2%. The constraining layer will furtler include a polyester resin such as the aforedescribed Dynapol S 320 resin in an amount of 0.5-5%. The constraining layer will also include a reactive diluent for viscosity control. This diluent comprises propylene glycol diglycidyl ether such as the material sold under the designation Erysis GE-24. This component is present in an amount of 5-15 weight percent and can react to incorporate into the polymer network of the material. The constraining layer also includes a polymerizable material such as epoxidized cashew nut shell liquid of the type sold under the designation Lepox 913. The epoxidized cashew nut shell liquid is present in an amount of 2-10% and polymerizes to enhance the corrosion resistance of the article upon which it is disposed. The composition further includes a crosslinker for the epoxy resin which, as previously described, may be dicyandiamide, which is present in this instance in an amount of 0.5-3.0%. The constraining layer will also include mineral components which comprise calcium carbonate in an amount of 10-50%, mica in an amount of 2-15% as previously described, as well as a titanium dioxide pigment in an amount of 0.1-1%. The mineral components also, in this instance, include fibrous materials which, in addition to functioning as fillers, enhance the integrity of the layer. These fibrous components include calcium metasilicate in an amount of 5-20%, and one grade of calcium metasilicate which may be used in the invention is commercially available under the designation Wollastonite M250. Another mineral component used in the composition is processed slag wool which is commercially available under the designation PMF fiber 204, and this component is present in an amount of 2-10%.

A number of specific materials may be prepared in accord with the foregoing. One particular composite, acoustic damping material is comprised as follows:

Extension (Bottom) Layer

Bisphenol A epoxy resin 5.0% Rosin ester 5.0% SBR rubber 10% Polybutadiene rubber 12.0% Diisodecyl phthalate 8.0% Process oil 8.0% Asphalt 2.0% Polyester resin 2.0% EMA copolymer 2.0% Sulfur 0.5% Dicyandiamide 0.5% Calcium carbonate 24.0% Barium sulfate 9.0% Mica 10.0% Fumed silica

Constraining (Top) Layer

Bisphenol A epoxy resin 15.6% ABS powder 8.0% Aromatic tackifier 1.0% Polyester resin 1.0% Calcium carbonate 35.0% Propylene glycol diglycidyl ether 12.0% Mica 5.0% Calcium metasilicate 9.0% Processed slag wool 5.0% Titanium dioxide 0.4% Epoxidized cashew net shell liquid 4.0% Dicyandiamide 1.0% Fumed silica

Both the material of the extension layer and the constraining layer had a density of 12.5 pounds per gallon, and both compositions are amenable to being applied to a body panel of a motor vehicle by automated application techniques.

Composite, acoustic dampening structures were prepared in accord with the foregoing, and their performance in a series of tests was compared with sound dampening structures of the prior art. Specifically, acoustic damping structures were prepared from the specific formulations detailed above. In this regard, standard automobile body panels having a thickness of 0.8 millimeters had a first, extensional layer in accord with the foregoing applied thereto. The thickness of the extensional layer was 3.0 millimeters+/−0.5 millimeters. Applied thereatop was a constraining layer as per the above formulation. The thickness of the constraining layer was 1.5 millimeters+/−0.5 millimeters. It was found that the foregoing composition bonded very well to oily metal, which is typical of the condition of in-factory automotive body panels. It also was found to bond very well to E-coated metal. The coated panels were subjected to a phosphate bath wash at 90° C., and remained intact. The coated material had a viscosity of greater than 1 mM cps; and even when coated at total thickness of up to 10 millimeters, no sag was observed on vertical surfaces. The coated panels were then subject to high temperature curing conditions which are simulative of those encountered in the course of painting and finishing a motor vehicle panel; and, it was found that the materials of the present invention are thermally cured and not degraded with regard to performance or appearance over a temperature range of 150-205° C. It is notable that this exceeds the high temperature limit for composite acoustic damping structures of the prior art. In this regard, compositions of the type shown in U.S. Pat. No. 4,346,782 have a maximum curing temperature of 170° C., and structures shown in U.S. Pat. No. 5,227,592 are limited to a temperature range of 100-180° C. As noted above, this high temperature capability of the materials of the present invention makes them very compatible with a broad range of painting and finishing processes.

The cured material was evaluated with regard to its ability to dampen sound, and it was found to perform at least as well as prior art materials in this regard. This is notable since prior art sound deadening materials such as the aluminum foil/polymeric sound deadening patches typically used in the prior art are generally twice as large in area as were the structures of the present invention. The sound loss factor at room temperature as measured by the Oberst bar technique was found to be in the range of 0.20-0.32 for the structures of the present invention. By comparison, prior art materials of the type shown in U.S. Pat. No. 4,346,782 had a sound loss factor in the range of 0.16-0.30 under the same test conditions.

The sound deadening ability of the materials of the present invention was measured over a frequency range of approximately 100-300 Hz by measuring the time required for “ring down” of vibrational peaks following the input of kinetic energy to a body panel (door slam test). In this regard, the materials of the present invention were compared with the prior art aluminum foil/butyl rubber patches presently used in the automotive industry, and as noted above, the patches of the present invention had an overall area which was less than one-half that of the prior art patches. Evaluations were carried out with both a rear door and a front door of a crew cab pickup truck, and it was found that over the entire frequency range, the performance of the patches of the present invention was at least as good as that of the larger area, prior art patches.

In a further series of evaluations, sheet metal body panel stock was coated with the patch structures of the present invention and subjected to various environmental tests. A first test was termed a “cold bend test”, and in it the coated panels were cooled to −40° C. and then bent to an angle of 180 degrees. The patches of the present invention remained intact and did not crack, separate or delaminate. In a second series of tests termed a “slam test” doors having the patches of the present invention adhered thereto were cooled to −40° C. and then slammed for 50 cycles. It was found that the patches did not crack, separate or delaminate. In a further test, the doors having the patches attached thereto were slammed, at ambient temperature, through 84,000 cycles without any adverse effect on the patches.

In a further evaluation, a series of patch structures of the present invention were applied to 0.8 millimeter thick steel panels. The patches were cured at a temperature of 210° C. and the panels were painted in accord with standard manufacturing procedures in the automotive industry. Thereafter, the surface quality of the painted panels was evaluated, and it was found that even with patch thicknesses of 6 millimeters, no read through was noted.

In summary, it was found that the patch structures of the present invention are economical and easy to utilize insofar as they can be applied to oily metal surfaces, utilizing robotic techniques implemented early on in a manufacturing process. The panels may then be subjected to conventional cleaning, painting and baking steps; and the patches of the present invention provide sound damping abilities which are at least as good as those of prior art patches, which typically are much larger. Thus, the use of the patches of the present invention results in a saving of material and labor in an automotive manufacturing process. The patch structures of the present invention are durable, reliable and provide long service life. Furthermore, the patch structures of the present invention do not include any foamed materials, and this prevents problems resultant from the absorption of liquids, as for example during subsequent washing, coating or storage operations. As such, the patch structures of the present invention represent a significant improvement over prior art structures and techniques.

As will be seen from the foregoing, the compositions of the present invention provide a very high degree of damping of a broad spectrum of acoustic vibrations over a temperature range corresponding to that normally encountered in the use of motor vehicles. In view of the teaching presented herein, the properties of the composite structure can be adjusted to accommodate specific temperate ranges as well as specific acoustic profiles, as may be encountered in various applications. For example, the composite structure of the present invention can be optimized for use in watercraft, home appliances, aircraft, static building structures and the like.

While the foregoing description has concerned structures comprising two layers of material, it is understood that the present invention is not to be limited thereto. In accordance with the present invention, multilayered structures may be likewise implemented. For example, a composite sound attenuating structure may include a plurality of stacked combinations of extensional layers and constraining layers. For example, a structure of 4-6 alternating layers of first and second polymeric material may be readily deposited, particularly by automated deposition techniques. Also, while the two layers are shown as being generally coextensive in FIG. 1, it is understood that other variations within the scope of the present invention here, for example, a sound attenuating structure may comprise an extensional layer of the first relatively soft polymer disposed on the article, and a constraining layer disposed on the first layer wherein the constraining layer is configured so as to cover only a portion of the first layer. In specific instances, the constraining layer may be configured as a series of strips or crosshatched members. In other instances, a continuous or discontinuous constraining layer may be interposed between subjacent and superjacent layers of relatively soft extensional material. All such embodiments are within the scope of the present invention.

In view of the teaching present herein, yet other modifications and variations will be apparent to those of skill in the art. The foregoing drawings, discussion and examples are illustrious of a specific embodiment of the invention, but are not meant to be limitations upon the practice thereof. It is the following claims, including all equivalents, which define the scope of the invention. 

1. A constrained layer, composite structure for damping acoustic vibrations in an article, said composite structure comprising: an extensional layer comprising, on a weight basis: 3-10% of an epoxy resin 3-10% of a rosin ester tackifier 5-12% SBR rubber 5-15% polybutadiene rubber 2-20% of a phthalate plasticizer 2-20% of an aromatic hydrocarbon plasticizer for said rubbers 1-5% asphalt 1-5% of a polyester resin (Dynapol) 1-5% of an ethylene methacrylate copolymer 0.3-0.8% of a crosslinker for said epoxy resin said first polymeric material having a first modulus of elasticity, wherein in the use of said composite structure, said extensional layer is disposed upon a surface of an article in which acoustic vibrations are to be damped; and a constraining layer comprising a second polymeric material which comprises, on a weight basis: 10-25% of an epoxy resin 5-15% of an ABS powder 0.5-2% of an aromatic tackifier resin 0.5-5% of a polyester resin 5-15% of propylene glycol diglycidyl ether 0.5-3% of a crosslinker for said epoxy resin said second polymeric material having a second modulus of elasticity which is greater than the first modulus of elasticity of said first polymeric material, said constraining layer being disposed atop said extensional layer.
 2. The composite structure of claim 1, wherein the first polymeric material further comprises, on a weight basis, 0.2-1% of sulfur.
 3. The composite structure of claim 1, wherein at least one of said first polymeric material and said second polymeric material further includes, on a weight basis, 1-40% of a mineral material, said mineral material being selected from the group consisting of fillers, pigments, thixotropy control agents, and combinations thereof.
 4. The composite structure of claim 1, wherein said second polymeric material further includes, on a weight basis, 2-10% of an epoxidized cashew nut shell liquid.
 5. The composite structure of claim 1, wherein the thickness of said extensional layer is in the range of 1-6 millimeters, and the thickness of said constraining layer is in the range of 0.5-3 millimeters.
 6. The composite structure of claim 1, wherein neither the first polymeric material nor the second polymeric material is a foamed polymeric material.
 7. The composite structure of claim 1, wherein neither the first polymeric material nor the second polymeric material includes polyvinylchloride or methylmethacrylate.
 8. A method for damping acoustic vibrations in a sheet metal body panel of a motor vehicle, said method comprising: applying said constrained layer, composite structure of claim 1 to said body panel.
 9. The method of claim 8, including the further step of painting said panel after said composite structure has been applied thereto.
 10. The method of claim 8, wherein the step of applying said composite structure is implemented utilizing a robotic applicator.
 11. The method of claim 8, including the further step of heating said panel to a temperature of at least 200° C. after said composite structure has been applied thereto. 